Production of an amino acid by utilizing a microorganism is known as amino acid fermentation, and is traditionally widely conducted in the field of applied microbiology. In amino acid fermentation, in the final step thereof, amino acid transport activity, that is, how to facilitate the efflux of the resulting amino acid to the outside of the cell of the bacterium is an important process that influences amino acid productivity; various attempts have been made to date to increase the efficiency of efflux to the outside of the cell.
Usually, active transport using bioenergy is necessary to transport an amino acid in the cell of a microorganism to outside of the cell. Proteins that export intracellular amino acids to outside of the cell (efflux proteins) have been identified; it is known that amino acid productivity can be also conferred or enhanced by enhancing the expression of the proteins. For example, a process for producing L-lysine using a strain of a microorganism of the genus Corynebacterium wherein the expression of the L-lysine, L-arginine efflux gene (lysE) (see non-patent document 1) is enhanced (see patent document 1); a process for producing L-cysteine, L-cystine, N-acetylserine or thiazoline derivative using a strain of a microorganism of the genus Escherichia wherein the expression of the efflux gene (rhtA) of L-threonine, L-homoserine (see patent document 2), and L-cysteine, L-cystine, N-acetylserine or thiazoline derivative efflux gene (ydeD/eamA) (see non-patent document 3) are enhanced (patent document 2); a process for producing L-amino acids, including L-lysine, using a strain of a microorganism of the genus Escherichia wherein the expression of the L-lysine efflux gene (ybjE) involved in L-lysine resistance is enhanced (patent document 3) and the like are known.
However, no report is available on an efflux protein for L-serine and L-glutamine, and a process for producing the amino acids wherein the activity of the protein is enhanced.
By the way, the Escherichia coli norM gene is known to be an efflux pump gene related to quinolone resistance (non-patent document 4). The emrD gene is reported as an SDS transport gene (non-patent document 5). While rarD is predicted to be a drug transport gene, none of them is known to have an amino acid efflux activity (non-patent document 6). Meanwhile, the eamA(ydeD) gene is reported as a gene having efflux activity for L-cysteine, L-cystine, N-acetylserine or thiazoline derivative (non-patent document 3).